Battery plate separator



T. E. PHlLlPPs BATTERY PLATE sEPARAToR Filed Dec. l28, 1949 sept. 29,1953

MATTE/er PLA ras? coA rml c accro/2 "6 BLADE Q47 rH//v MA r THICK MA T Patented Sept. 29, 1953 BATTERY PLATE SEPARATOR Thomas E. Philipps, Granville, Ohio, assigner toy Owens-Corning Fiberglas Corporation,A a cor,-

poration of Delaware Application December 28, 1949, Serial'No'. 135,486

15 Claims.

This invention relates to interlayers or mats used to separate the positive from the negative plates of electrolytic cells ofthe type which make up a storage battery. The application is a continuation-in-part of my application filed December 31, 1948, Serial No. 68,600, now abandoned.

The ordinary storage battery is constructed with a plurality of such electrolytic cells formed of a series of connected positive plates and a series of connected negative plates which are interleaved to arrange the plates in closely spaced parallel relation. The plates, formed of leadantimony alloy grids to which a paste of active material is loosely bonded, are immersed in an electrolyte usually constituted of dilute sulphuric acid.

The useful life of an electrolytic cell is ended when conductive contact is established between the positive and negative plates. Contact of the type described may result from a large number` of causes incident to the normal reaction which takes place during charge and discharge of the cell or battery. Frequently, one or both of the plates become buckled, while in position of use, to the extent that actual plate to plate contact is established. Very often the plates become bridged at one or more spaced apart points by metallic deposits or deposits of active material which build up on the walls of the plates, by inadvertent depositions or by other phenomena, i

such as is often referred to as treeing The commonest device to cope with these problems is the practice of inserting a separator or spacer between the plates to operate as a barrier to the build-up of trees andto minimize or even prevent buckling.

The qualiiications of an acceptableseparator have been established through extensive usein various types of electrolytic cells operating on the wet-plate principle; it should have sufficient mass integrity and rigidity to carry itself between the plates and to hold the plates in the desired spaced relation; it should be inert with respect to the electrolyte, and it should not contaminate the electrolyte to the extent of harmully affecting the reactions of'the current generating system; it should not create local reactions with the plates whereby their decomposition is accelerated or whereby effective use is not'. made of the maximum available area; it should be porous to permit free circulation of the electrolyte whereby improved performancev isV secured by raising the capacity of the cell at a given discharge rate and to permit the rise of any generated gases; and

it should prevent passageof materials. such. asi

is often referredto in the art as mud. These characteristics are not completely satisfactorily met by grooved but not porous or sufficiently porous plates of wood, glass, or rubber often used in battery assemblies. Nor are they satisfactorily met by the more recently developed molded ebonite plates.

Anotherv factor which inuences the life and utility of the cell is the rate of loss of active material from the face of the plates, especially from the face of the positive plate to which it is weakly bonded. To minimize this loss, it has been the practice of the industry to insert a retainer sheet between the separator and the respective plate. To accomplish its purpose in a satisfactory manner, the retainer should be suffi.- ciently porous to permit gases to travel therethrough and to permit the free circulation of the electrolyte; it should contact the plate over substantially its entire area to hold the paste of active materialk in place, and it should be inert with respect to the materials with which it is associated to theextent of. resisting attack by the electrolyte, not contaminating the electrolyte, ynor setting up local reactions with the plates.

It is an object of. this invention to produce mats for use between positive and negative plates of an electrolytic cell which are formed of glass ber fabrics and perform both the functions of a separator and retainer.

Another object is to produce a porous fabric constituted essentially of glass fibers which meets substantially all of the qualifications of a good battery separator and retainer.

Most ofthe desirable characteristics for a mat between positive and negative plates are met by the use of a porous fabric of. glass fibers, except that the porous fabric still permits treeing to bridge the plates. Therefore, another very important object of my invention is to produce a porous mat of the type described which is formed with a barrier constituted of materials and an arrangement of materials that prevents treeing without detractingA from any desirable properties resulting from the use of a porous fabric of glass iibers.-

These and other objects and advantages of this invention will hereinafter appear and for purposes of illustration but not of limitation, embodiments are shown in the accompanying drawings, in which:

Figure l is a perspective view of a portion of ai storage battery embodying'the present inven- Figure 2 isa schematic view of one form of apparatus for the production of a structure ernbodying the invention;

Figure 3 is a schematic view of a modified form of apparatus;

Figure 4 is an enlarged sectional view of a stucture formed by the method illustrated in Figure 2;

Figure 5 is a similar view of a separator formed by the method illustrated in Figure 3; and

Figure 6 is a highly magnified view of the layer material used on separators of the present invention.

In the layer material for separators of the prior art, the binder was intimately mixed with the layer composition. Instead of forming particles, the binder formed films which were substantially continuous throughout the separator. Naturally, this greatly reduced the porosity. In order to obtain some desired degree of porosity, it then frequently happened that the materials used were made less dense. Such a change, however, only increased the opportunity for treeing, and the like. Consequently, the goals of desirable porosity and resistance to treeing seemed inconsistent and irreconcilable.

It has also been proposed to make a battery separator comprising a porous sheet, such as of glass fibers, having a slightly porous sheet of synthetic plastic material as a coating. The coating is designed to reduce the porosity of the first sheet and thereby also reduce treeing or bridging But here, as before, a reduction of porosity only introduces new problems. As an example, circulation of electrolyte becomes increasingly diiiicult. Also, gas bubbles formed during the discharge of the battery cannot readily rise and escape, resulting in a lowering of battery efficiency.

The present invention is based upon the discovery of a greatly improved interlayer or battery separator characterized chiefly bv increased porosity without a sacrifice of any of the previously mentioned desirable properties, such as resistance to treeing More particularly, the present invention provides an interlayer or separator comprising a porous mat, preferably made of glass fibers, having a layer of an impregnating composition comprising a combination of minute, distinct, separate particles or pebbles.

Some of the particles serve to bind the other particles one to another to create an integral mass. Thus, a structure is obtained in which the particles or minute pebbles make substantially only pin-point contact with each other. Consequently, the area surrounding each point contact is open for the free passage of fluids. As a result, desirable porosity is easily obtained by the present separator and yet there is not a sacrice in density or the like which make the separator susceptible to treeing and bridging Nor is there a consequent loss of any of the other previously mentioned desirable properties.

The layer particles may be made of any material which is inert to battery action and capable of reduction to minute particle form. The size of such particles may vary Widely depending on the final properties desired. As an example, a satisfactory diameter size may range from 10 to 80 microns, although other sizes may be used.

Materials which are or can be comminuted to form layer particles of the present invention include siliceous matter comprising silica, such as ordinary, substantially iron-free sand; and

silicates, such as aluminum silicate, pyrophyllite, Wollastonite, pumice, and substantially iron-free clays. Silica is preferred since it is easily obtained in a finely divided state and is very inert to the chemical action of a battery.

A highly preferred form of silica is diatomaceous earth, the material composed of skeletons of the diatoms and like unicellular alga Whose walls are thought to have acquired silicon through chemical double decomposition with siliceous matter present in the same environment.

The particles of diatomaceous earth are each composed of a multiplicity of tiny, petrified skeletons of the diatoms. Consequently, each particle of the skeletonized diatoms is itself full of open cavities and pores.

It has been estimated that forty million of a given species of these fossil remains of unicel lular plants may be contained in one cubic inch. Yet microscopic examination shows each one to be delicately fashioned into a. highly ornate structure with numerous channels, perforations and nodules, all combining to give extensive sur-l face area. The diameter or longest dimension of such particles varies from one to microns. An average is 25 microns.

As a result, a separator of the present invention utilizing diatomaceous earth and made according to the present invention has excellent porosity since paths of flow are available through as Well as around these particles. Yet the openings in the skeletonized diatoms are not suiiiciently large to encourage treeing, bridging, and the like.

Over 10,000 varieties of diatoms are known, so no attempt is made to list possible choices. However, examples of forms or names by which these skeletonized diatoms are known include diatomaceous earth, siliceous earth, infusorial earth, kieselguhr, diatomite, tripolite, ceyssatite, guhr, bergmehl, fossil flour, farine fossil, and tellurine. They are also obtainable under the trade names "Celite and Dicalite The binder particles which serve to bind the layer particles into an integral mass may comprise elastomers such as butadiene-styrene copolymers (GRS or Buna S), natural rubber, butadiene-isobutylene copolymers, polyisobutylene, organo-silicon elastomers, butadiene-acrylonitrile copolymers, and chloroprene polymer.

The elastomers may be applied as hereafter shown from a non-solvent suspension or emulsion, for example, from an emulsion like natural latex wherein the rubber is present as minute, distinct separate particles.

Elastomeric b-inder particles are preferred because separators having this type of binder are much more flexible and resistant to handling and cracking on bending. If these properties are not essential or desirable, it is also possible to use other types of resins for the binder particles. For example, thermosetting resins such as phenol-formaldehyde and thermoplastic resins such as plasticized polystyrene, plasticized polyvinylidene chloride, polymethyl methacrylate, and the like may be so used when suspended, for example, as particles in a non-solvent medium such as Water. In the case of thermosetting resins, the particles are preferably partially cured.

The binder particles are generally supplied from a suspension or emulsion having l0 to 60 per cent solids content by Weight. Of the binder particles mentioned, the nitrogen-free and chlorine-free materials are preferred to avoid the possibility 0f. any deleterious eiTect on battery action.

As shown in Figure 6, the binder particles 29 hold the :layer particles 30 together -to .for-m the porous layer 3l for theglass i-berfmat previously described. The .binder thus composed of :subdivided, :sepa-rate little ,particles which make fsurtace and possi-'bly 4.only lpoint contact with the layer particles, much as the grain-s ldo in ,a body of sand. As a consequence, there is opportunity .for-.pores -32 to exist educato the more or .less point ,contact between the binder particles and layer particles, Aand :therefore a :resultant free and easy .fluid flow around the layer particles. Particularly fin the :case of the rskeletonized diatoms, the binder particles :are 1 especially meritorious :since the pores .and channels `of the diatoms .are met illedfor'iilmed over..

ln `carrying out 'the invention, l provide a :bat- .tery plate `separator r|10 Ahaving the #semblance of a :laminate formed with fa relatively thick highly porous layer III of bonded glass fibresfand a thinner microporous layer -t2 :on atleast one face of the highly porous layer` The microporous layer .is iformed of ya composition l'3 .including :finely rdivided silica, or silicate particles .bonded 'by binder .particles to Acreate anintegral mass .and carried :by :a )thin sheet or mat M `of glass bers. 'The sheet M may be :a glass 'fiber fabric formed of interbonded haphazardly varranged fibers, the pores of which normally are dimensioned to allow easy circulation of Ithe electrolyte and escape of battery gases.

`In .a .specific application, the microporous .layer l2 'is separately formed `by impregnating `a ,thin sheet, such as a 10 mil thick bonded glass ber mat M, with a composition or paste comprising an elastomeric bonding agent in particle jforrn that is inert with respect to the electrolyte or battery acids, and iinely divided silica, or silicates. When 'the consistency of the vpaste Ais `too light for proper impregnation or application to fthe sheet, or in the event that the paste is an yacueous suspension or emulsion, a -bodying agent may be incorporated into the paste composition, and `the use lof wetting agents lmay lend `*many benecial eifects.

Impregnation of the sheet y'or mat M is -calculated to build vup a fmi'croporou's 'la-yerto Ya vthickness of about l0 to 40 -Inils on a 5 to 15 mil sheet. Before the impregnating composition on the sheet has had ran opportunity to ldry, that is, lwhile it is still in a somewhat tacky stage, the impregnated sheet is joined to a porous mat of bonded glass fibers corresponding, in this illustration, 'to a el) mil mat bonded with an adhesive such Jas Aa .cured phenol formaldehyde resin.

Thereafter, Ias the 'diluent is removed `from the impregnating composition, the binder particles adhere the layer particles one to another to create an integral mass Ahaving interstitial spaces as described which are dimensioned to limit passage chiefly to electrolyte and gases while .preventing treeing or 'the passage of"m ud:

In thepresent invention, .the manner of removing the diluent can be critical. Ordinarily, .the .porous mat with its impregnated sheet yis passed through a heating means, such .as an oven, to hurry the removal or evaporation of the diluentl In such cases, it is desired to confine this operation to as `'short a time as possible and, ,accordingly, the temperature of exposure is customarily as high as thematerials can kstand `or economics allow.

However, l .have found that ,there is a fairll7 definite, `critical temperature .above which separators Aof the `present vinvention cannot -be heat- :ed duri-ng :the drying operation .if elastomers are fused-.as *the .binder and if .all advantages of the .invention are to .be realized. Nonobservance of this :limitation ,fluidizes the .elastomer ybase yor ,binder :to such an 4extent that it .loses its subdivided, separate particle shape. Instead, the elastomer :becomes 4film-forming, and ein this condition tends to cover zthe .entire surfaces of the layer particles, thereby reducing the `size of the :openings :between lthe particles and materially reducing the porosity. This is particularly serions in the .case :of diatomaceous earth since the .pores and .channels -are filled or ffilmed over.

A secondary :objection to vfilm-forming is that it actually `hinders y:removal of the diluent. AFor example, separators heated -above the film-forming ytemperature of .the elastomer .used are Afound to contain morediluent vthan those heated below rforathe fsame period of-time.

Actually, whether ,elastomers or other resins ,are used :as the binder particles, the temperature `arf-.the :drying `operation need be only sufficiently higher thanfthetemperature of the separator to form a thermal potential and encourge evapora.- tion of kthe diment. Or fthe diluent maybe re- :moved under sub-atmospheric pressures. However, the time involved especially in the rst instance, maybe too long to 'be Veconomically feasible. Therefore, the "lowest practical temperature of exposure fis in :the neighborhood of v125" F. Time .of :exposure atthis temperature may average :about :anhour or longer.

:In fact, in the Vcase of the elastomer binder particles, it fis preferable .to fheat the separators ata temperature "suiiicient to render the elastomer at leastsemi-tacky. This has been found to result in :a iirmer bonding action and *to overcome a tendency Iof the elastomer particles to crumble l'andl'eave -the separator, Vespecially-upon striking lbattery electrolyte, iffnot so treated.

For fthe most part, elastomers in general, and particularly those disclosed herein as possible `'choicesvnoay"be made atleast semi-tacky without becoming nlm-forming 'if vheated in the temperature Vrange .'ofabout 200 'F'. to `220" F. Of course, some elastomers 'may lb'e heated at a still khigher Atemperature lto reduce the time needed for drying. 'For examplegthe preferred elastomer, butadiene-styrene, oopolymerized iin approximately a twefto 'three molecular ratio, respectively, can be Aheated as ihigh 4as about S7-5 'without lming.

In any case, if there is doubt of the tempera.- ture at which an elastomer may film, an allowfable temperature of lexposure can be simply determined Lby u'heating for the `same vtime a Iseries yof separa-tors containing'the elastomer in question at as vmany different temperatures in the Aneighborhocd vof fthe desired drying temperature, then determining 'theporosity of each, and 0bserving for what temperature of exposure the porosity begins `to decrease. This temperature indicates when iilmng has vbegun and therefore a lower temperature, for example, that of the separator next 'lower in temperature in the test, ishouldbe used 'in practice.

As one method of determining or indicating porosity, 'the electrical resistance through a separator when immersed in electrolyte may be noted vbylstandard 4procedures used in the trade. If an increase Lin ,resistance is found in a series ofseparators 'treated at progressively higher temperatures, .thelming temperature of the elastomer has been ,reached or passed by that separator .showing .the ,increased resistance.

The 'temperatures at which other resins .such

as the phenolics may be heated when used as the binder particles are well known and are not critical in the practice of the present invention. As an example, semi-cured phenol formaldehyde particles may be polymerized to the nal stage at 275c F. or higher. Thermosetting resin binders are preferably applied in the B stage of polymerization to prevent their filming during the subsequent heating step.

With respect to the removal of the diluent, the drying operation need be continued only until a substantial amount has been removed. By substantial is meant of the order of 90% or more. Complete removal of the diluent is unnecessary since at any rate the separator tends to absorb moisture from the atmosphere if absolutely dry. As an example, one exposure of a separator containing butadiene-styrene particles at 380 F. for ve minutes removed about 98% of an aqueous diluent. Upon the removal of the diluent which was included in the impregnating composition, the composition will be of a microporous character Ywhich is supported and reinforced by the glass ber sheet.

The mat II may be formed in the manner described in the Slayter Patent No. 2,306,347 or in other desired ways.

The binder of the highly porous mat I I and the thin bonded or woven sheet I4 is selected of those resinous or rubber-like materials strongly adherent to the glass fiber surfaces and impervious to the battery acids and electrolyte. In addition to phenolics such as phenol formaldehyde, other known binders may be used. Preferably the binder should be nitrogen-free to avoid any possible adverse eiTect on the battery action. Improved binders for mats to be used in electrolytic cells of the type described are selected from the polyacrylates such as Dolymethyl methacrylate or polystyrene having an average molecular weight in excess of 65,000. When the binders are applied from aqueous system, and particularly when polystyrene comprises the bonding agent, additional advantage is derived from the use of a small amount of gelatin or other like protein in amounts less than one-fourth the weight of base resin. Gelatin functions to stabilize the emulsion or dispersion, but more irnportant, it is immediately operative as an adhesive temporarily to bond the various bers until n the base resin becomes effective for adhesive purposes.

Very often the binder is applied from solvent solutions of to 40 per cent by weight, in which instance mere evaporation by air, dry or with the aid of elevated temperatures, may be suicient to set the resinous materials. More often the binder is applied as an aqueous emulsion or dispersion of the type described whereby the resinous materials are deposited as discrete particles on the liber surfaces and it is necessary to raise the temperature ot the particles above their fusion temperature to carry them through an adhesive stage. it is in such aqueous emulsions or dispersions that gelatin or such materials iind best use as an intermediate binder until the resinous bond is developed. The amount of binder ordinarily employed varies according to the materials of which it is composed and the type of mat or the arrangement of the fibers in the fabric. Very often suincient mass integrity is secured by the use of 10 or 20 per cent binder calculated on the of solids by weight, but ordinarily best results are secured when the binder content of the mat is above 30 per cent but it seldom rises above 50 per cent.

which are With respect to the amount of materials used in the microporous layer I2, the ratio of the binder particles to the layer particles may range from 1:1 to 1:4 parts by weight, respectively. Best results are secured when the materials are present within the ratio of 1 part binder particles to 1.5 to 2.5 parts by weight of layer particles.

In aqueous medium, the bodying agent, when used, may consist of gelatin, ammonium alginate, alkali metal alginates or other water soluble resinous or high molecular weight materials. Less than 3 per cent bodying agent is suicient, but as much as 5 per cent may be used.

Suitable wetting agents may be selected from the group of materials consisting of the dioctyl esters of sodium sulfosuccinate (Aerosols), substituted aromatic sulfonates (Duponols), dibutylphenol sodium disulfonates (Areskelene), sulfonated ethers (Tensol), and the like. Less than 2 per cent of the wetting agent ordinarly is sufficient to impart the desired wetting characteristics, although more may be used when desired.

The following examples of suitable impregnating compositions or pastes and their methods of application in the manufacture of a combine separator and retainer are given by way of illustration and not by way of limitation.

Immer/noting compositions EXAMPLE 1 20 parts butadiene styrene copolymer aqueous suspension). l0 parts Dicalite.

EXAMPLE 3 40 parts natural rubber latex (50% emulsion). 10 parts aluminum silicate. 20 parts silica (iron-free). 10 parts ammonium alginate (in 20% solution).

EXAMPLE 4 40 parts plasticized polystyrene (95,000 MW) (40% aqueous suspension).

20 parts gelatin (4% solution).

0.5 part Aerosol OT (wetting agent).

30 parts clay.

In making the impregnating compositions, the

indicated ingredients are merely mixed together preferably with stirring.

Preparation EXAMPLE 5 Figure 2 of the drawing illustrates one method of manufacture in which the thin mat I4 is advanced from a roll I5 to receive a deposit of compound I3 from a coating device I5. Doctor blades I'I serve to smooth the layer of compound and work it into the interstices between the bers of the mat. Before the composition has had an opportunity to dry, the thicker mat II is superimposed and the layers are advanced together between cooperating rolls I8 and I9 which insures their integration. The integrated layers or laminate is carried through a heating over 20 maintained at a temperature within the range of F. to 375 F. to accelerate the removal of the diluent, which if more time is available 9 might also be effected by a simple air dry. It will be apparent that integration or lamination results from adhesion inherent in the impregnated or coated sheet, and that in the integration there will be little tendency for the composition coating to penetrate the porous mat Il. In the alternative, the two layers may be combined by the separate application of an integrating adhesive to one or both of the layers before integration. This process produces a product which corresponds to that shown in Figure 4.

EXAMPLE 6 Figure 3 illustrates another technique for fabricating a combined battery plate retainer and separator. By this method the thin mat i4 is impregnated and coated by a roller coating process, indicated schematically by the numeral 2l. The impregnated mat converges toward the thicker mat ll fed from a. roll 22 and the two are integrated in superposed relation between rollers 23 and 24 and then advanced through the drying oven 25. The product of this process, illustrated in Figure 5, is a laminate having a relatively thick layer represented by the mat Il and another layer which may be divided into three parts including a layer 26 of compound reinforced with the mat i4 of glass ber and sandwiched between layers 21 and 28 of compound free of reinforcing glass fibers.

The thickness of the combined separator is ordinarily selected to correspond dimensionally to the span existing between the positivek and negative plates of the cell. In most units where the distance corresponds to about 70 mils, the highly porous mat is about 40 mils, but it may be selected of mats ranging from 30 to 60 mils thickness. Correspondingly, the microporous layer may range from 40 mils to 10 mils in thickness. It is preferable, however, to have the highly porous layer Il of greater dimensional thickness than the microporous layer I2.

Ordinarily it is sufcient if the microporous layer is disposed on only one side of the highly porous layer where it is adapted to hold the paste on the positive plate in position. However, when desired, microporous layers may be arranged with the highly porous layer sandwiched therebetween.

It will be manifest that I have produced as my invention, a battery plate separator which incorporates a retainer member and the separator in one and the same unit; a separator which is highly porous to permit the desired circulation of electrolyte and upward passage of gases and, at the same time, provides a barrier to treeing and minimizes the loss of active material from the face of the plates; and a separator which is impervious to the electrolyte and does not cause local reactions with the battery plates.

It should be understood that the article produced by my invention comprises a new and improved laminate which may be used as a plate separator in other types of cells, and which may be used for many other purposes. It will be further understood that numerous changes may be made in the details of construction, arrangement, materials and their concentrations without departing from the spirit of the invention, especially as defined in the following claims.

Iclaim:

1. A combined separator and retainer for storage batteries comprising a mat of glass bers mechanically bound at their junctures by a resin and an adhesive microporous composition providing a continuous, non-impregnating cover on at least one side of the mat, said composition comprising diatomaceous earth particles and unvulcanized butadiene styrene particles which were polymerized approximately in the molar ratio of 2:3, respectively.

2. A combined separator and retainer for storage batteries comprising in laminar relationship a mat composed of haphazardly arranged glass fibers which are-'mechanically bonded at their juncture by phenol formaldehydepolymer, a microporous inherently adhesive layer covering one side of the mat without impregnating it, said layer comprising diatomaceous earth particles bonded one to another by particles of an unvulcanized 40:60 butadiene styrene copolymer, the ratio of these particles to the diatomaceous particles ranging from 1:1 to 1:4 parts by weight respectively.

3. A method of making a combined separator and retainer for storage batteries comprising flowing an aqueous composition of diatomaceous earth particles and unvulcanized butadiene styrene copolymer particles onto a mat approximately 30 to 60 mils thick and composed of haphazardly arranged glass fibers which are mechanically bound at their junctures by phenol formaldehyde polymer, spreading the composition vover the mat vWithout impregnating it to form a. continuous covering about 10 to 40 mils thick, and then heating the assembly up to 375 F. to bind the diatomaceous earth particles to each other through the butadiene styrene particles and to adhere the composition to the mat, the butadiene styrene polymer particles being prepared by reacting butadiene and styrene in approximately a molar ratio of 2:3 respectively, the ratio of these particles to the diatomaceous particles ranging from 1:1 to 1:4 parts by weight respectively.

4. A combined separator and retainer for storage batteries comprising in laminar relationship a thick mat comprising haphazardly arranged mineral bers which are mechanically bonded at their junctures by a phenolic resin, a microporous inherently adhesive layer covering one side of the mat without mpregnating it, said adhesive layer comprising diatomaceous earth particles bonded one to another by particles of an unvulcanized 40:60 butadiene styrene copolymer, the ratio of these particles to the diatomaceous particles being 1 :2.

5. The article of claim 4 wherein the ratio of butadiene styrene copolymer particles to the diatomaceous particles is 1: l.

6. The article of claim 4 wherein the ratio of butadiene styrene copolymer particles to the diatomaceous particles is 1 :4.

'7. An interlayer for use between battery plates comprising a. porous mat of bonded glass bers having an adhered, contacting microporous layer on at least one face of the mat consisting essentially of a finely divided silica, an unvulcanized butadiene styrene elastomer bonding agent which is not attacked by the battery acids, a thickening agent, and a wetting agent, said layer being reinforced by a glass fiber sheet.

8. An interlayer as claimed in claim 7 in which the layer materials are present in the ratio of about 1 part by weight of the elastomer material and l to 4 parts by weight of the finely divided silica.

9. A battery mat comprising a pair of porous glass ber mats, the fibers of each mat being bound to adjacent bers lat their junctures, said mats having different thicknesses and held in a laminar relationship by a microporous composition which completely impregnates and covers both faces of the thinner mat and adheres it to a major face of the thicker mat, said composition comprising siliceous particles suspended in an unvulcanized elastomer bonding medium impervious to battery acid.

10. A separator as claimed in claim 9 wherein the siliceous particles are diatomaceous earth land the elastomer is butadiene-styrene copolymer.

11. The combined separator and retainer of claim 2 wherein the ratio of the unvulcanized butadiene styrene copolymer particles to diatomaceous particles is 1 :2.

12. The combined separator and retainer of claim 2 wherein the ratio of butadiene styrene copolymer particles to diatomaceous particles is in the range from 1:1.5 to 1:2.5 parts by weight.

13. A method of making a combined separator and retainer for storage batteries comprising owing an aqueous composition of diatomaceous earth particles and unvulcanized butadiene styrene copolymer particles onto a mat composed of haphazardly arranged glass bers which are mechanically bound at their junctures by phenol formaldehyde polymer, spreading the composition over the mat without impregnating it to form a continuous covering, and then heating the assembly up to 375 F. to bind the diatomaceus earth particles to each other through the butadiene styrene particles and to adhere the com- 12 position to the mat, the ratio of these particles to the diatomaceous particles ranging from 1:1 to 1:4 parts by weight, respectively.

14. The method of making a combined separator and retainer of claim 13 wherein the ratio of butadiene styrene polymer particles to diatomaceous particles is 1 :2.

15. The method of making a combined separator `and retainer of claim 13 in which the ratio of butadiene styrene particles to diatomaceous particles is from 1:1.5 to 1:2.5.

THOMAS E. PHILIPPS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,526,942 Steerup Feb. 17, 1925 2,043,954 Kershaw June 9, 1936 2,117,371 Slayter May 17, 1938 2,155,016 Kershaw Apr. 18, 1939 2,175,798 Hauser Oct. 10, 1939 2,306,781 Francis, Jr Dec. 29, 1942 2,397,453 White et al Mar. 26, 1946 2,428,470 Powers Oct. 7, 1947 2,478,186 Gerber Aug. 9, 1949 2,484,787 Grant Oct. 11, 1949 2,526,591 Szper Oct. 17, 1950 FOREIGN PATENTS Number Country Date 646,166 Germany June l0, 1934 451,628 Great Britain Aug. 10, 1936 537,377 Great Britain June 19, 1941 

1. A COMBINED SEPARATOR AND RETAINER FOR STORAGE BATTERIES COMPRISING A MAT OF GLASS FIBERS MECHANICALLY BOUND AT THEIR JUNCTURES BY A RESIN AND AN ADHESIVE MICROPOROUS COMPOSITION PROVIDING A CONTINUOUS, NON-IMPREGNATING COVER ON AT LEAST ONE SIDE OF THE MAT, SAID COMPOSITION COMPRISING DIATOMACEOUS EARTH PARTICLES AND UNVULCANIZED BUTADIENE STYRENE PARTICLES WHICH WERE POLYMERIZED APPROXIMATELY IN THE MOLAR RATIO OF 2:3, RESPECTIVELY. 